[Abstract] : This paper introduces the structure and characteristics of the SLG500 series PLC and, combined with its application example in the temperature control system of microbial cultivation in soy sauce production, demonstrates that applying PLC to the temperature control system can make the system have advantages such as simple structure, high reliability, superior performance, and ease of use. Keywords : PLC, closed-loop control, variable frequency speed regulation 0 Introduction PLC can realize closed-loop control of analog quantities such as temperature, pressure, and flow. It collects data through AID, outputs control quantities through D/A, and adds control algorithm modules (such as PID control) to form a high-precision closed-loop control system. This system can make these controlled analog quantities change according to the expected requirements and has the ability to communicate with the host computer. It can form dual-machine, multi-machine control systems and larger distributed control systems. 1 System Hardware Structure The principle block diagram of the soy sauce microbial temperature control system is shown in Figure 1. In this system, most of the equipment uses Rockwell products, among which the PLC adopts the SLC500 series, including a power supply module, a CPU module (with built-in ETHERNET interface), a temperature detection module (RTD), and a scanning module (with built-in REMOTE I/O interface). The host computer is a microcomputer (PC), and communication between the CPU and the host computer uses industrial Ethernet (ETHERNET). The frequency converter is a PowerFLEx700 series with a rated power of 4kW. The temperature sensor is a Pt100 RTD. The temperature signal transmission cable is an RVVP 3×1mm shielded cable. (Full text available for download: PLC Application in Temperature Control System)